1. Technical Field
The present invention relates to a rotational recording apparatus, such as a hard disk drive (hereinafter referred to as an HDD), and a control method therefor. More particularly, the present invention relates to a technique for applying feed forward control, using an adaptive filter, to control the position of an HDD head for which a shock sensor is provided.
2. Description of the Related Art
Recently, as the memory capacity provided for recording media for rotational recording apparatuses, especially HDDs, has been increased, the track widths employed for data recording have narrowed. As a consequence of this reduction in track widths, a need has been established for stricter control of the level of the disturbance produced mechanical vibrations to which a head is subjected. That is, since the probability has been increased that the offtracking of a head will occur from disturbance by vibrations that would not affect the head when the track width was satisfactorily large, a demand exists for means to limit head vibration, due to the disturbance, to an oscillation range that is appropriate for the track width. While head offtracking during data reading is a problem, because of data reading errors that occur, head offtracking during data writing is an even more critical problem. When offtracking occurs during the data writing process, not only is data not written to an intended track, but data on an adjacent track may be destroyed. Therefore, the prevention, to the extent possible, of head offtracking is imperative.
For the prevention of head offtracking due to a disturbance, a technique is well known whereby a shock sensor, such as an acceleration sensor, is provided for a rotational recording apparatus, such as an HDD, and the shock sensor output is employed for head positioning. For example, a technique is disclosed in Japanese Unexamined Patent Publication No. Hei 6-333325 (document 1) whereby the output of an acceleration sensor is passed through a filter, which is obtained using the transfer function of a head positioning control system, to calculate the position of a head, and when the position of the head exceeds a predetermined positioning range, data writing is inhibited. According to this technique, when sensor output is obtained indicating that head offtracking can occur, writing is inhibited, so as to prevent, for example, the destruction of data along an adjacent track.
Another technique, disclosed in Japanese Unexamined Patent Publication No. Hei 7-130114 (document 2), provides for sensor output to be transmitted to a filter having the same dynamic characteristics as the transfer function of a system for transmitting an acceleration disturbance received by a head actuator, and for a feed forward control signal to be generated that is employed to cancel out a disturbance produced vibration that can affect head positioning. That is, a filter is obtained in advance that can simulate the movement of a head due to a disturbance, and a signal output by the filter that receives the sensor output is employed to exercise the feed forward control. According to this technique, when a disturbance acts on a head, quick head positioning control can be implemented, and the occurrence of head offtracking can be prevented.
However, the following problems are inherent to the conventional techniques described above. Specifically, according to the technique in document 1, the occurrence of offtracking is predicted, and when the probability of the occurrence of offtracking is high, serious damage can be prevented by inhibiting data writing. However, this technique does not suppress the vibration of a head due to a disturbance, and can not prevent offtracking. In other words, this technique merely erects an individual barrier that will prevent the occurrence of an undesirable event when the head is off the track. For example, when a continuous disturbance, such as a cyclic vibration, acts on the head, normal data writing can not be performed while the disturbance continues.
According to the technique in document 2, it is ensured that at a head position the vibration due to a disturbance will be suppressed. However, this technique is based on the assumption that a filter has been obtained for reproducing the movement of a head when a disturbance occurs. At this point, a problem arises. That is, although the movement of a head can be precisely reproduced when appropriate sensor output is available, it is generally difficult to obtain in advance a relationship that will provide the needed output.
Since a system may change dynamically, adaptive signal processing using an adaptive algorithm is a well known method for optimally controlling a system wherein a parameter can be varied. Means can be employed by which the adaptive algorithm is employed to obtain the filter coefficient used in document 2. In accordance with the adaptive filter that is used for this adaptive algorithm, it is anticipated that the optimal parameter will be obtained while a feed forward signal that implements the optimal head position control using the sensor output is generated. However, the following problem is encountered with this method.
FIG. 8 is a block diagram showing an example conventional control system for adaptively performing the feed forward process using a signal output by a shock sensor. An HDD is employed as an example. When G denotes the transfer function of a VCM (Voice Coil Motor) system extending from the transmission (input point I) of a current for a VCM to the head position (deviation d from a target value), and H denotes the transfer function of a controller extending from the head position deviation d to a feedback control signal fb, the output (deviation d) of the transfer function G is fed back through the transfer function H to the input point I. On the other hand, a disturbance ef acts on a mechanism MS to generate a head position deviation def that causes a disturbance, and also acts on a shock sensor to generate output s. The sensor output s is passed through an adaptive filter FIR, and is transmitted to the input point I as a feed forward signal ff. The parameter of the FIR is then determined by a parameter adaptation algorithm PAA, so that the feed forward signal ff cancels out the head position deviation def that caused the disturbance. To determine the parameter, the head position deviation d and the sensor output s must be referred to; however, since the position acted on by the FIR output (feed forward signal ff) differs from the position acted on by the head position deviation d, phase compensation is required. A filter that controls the phase compensation is a phase shift filter, and this phase shift filter is the one that approximates G/(1+GH).
The problem with this technique is that the phase shift filter must approximate G/(1+GH) for the entire frequency band. And when the order of the phase shift filter is raised, approximating is especially difficult. If the phase shift filter can not approximate G/(1+GH) in the operating frequency band, the stable operation of the PAA is not guaranteed, and the normal operation of the control system in FIG. 8 can not be expected.
It is, therefore, one object of the present invention to provide a rotational recording apparatus that comprises a shock sensor and that adaptively processes the output of the shock sensor to obtain a feed forward signal, so as to implement a stable operation within a practical range.